Process for continuous xanthation and production of viscose spinning solution

ABSTRACT

A continuous feed of alkali cellulose (AKC) is partially xanthated, and then dissolved and further xanthated to form a viscose solution, while passing continuously through a series of connected reactors and slurry vessels, wherein slurries of carbon disulfide (CS2) and alkali metal hydroxide are maintained. The AKC feed to the system may be shredded crumbs from any standard alkali cellulose system, or shredded crumbs from a continuous sheet steeping mercerization system. A portion of the xanthate AKC (cellulose xanthate) may be recycled to the various slurry vessels or zones.

This invention pertains to process and apparatus for continuouslyxanthating alkali cellulose (AKC) and for converting such alkalicellulose into a spinnable viscose solution.

In many conventional processes for making viscose spinning solution,alkali cellulose is xanthated by contact with carbon disulfide (CS₂) ina batch process. Typically, the alkali cellulose is in crumb or shreddedform and is converted into a sticky mass by xanthation.

Numerous efforts have been made to provide a commercially acceptablecontinuous xanthation process and apparatus therefor. Examples includethe processes and apparatus disclosed in U.S. Pat. Nos.2,762,795-Schlosser et al (xanthation of sheet alkali cellulose passingthrough a sealed chamber), 2,801,998-Robertson (conventional crumbxanthation followed by later stage involving continuous mixing in wetforms), 2,987,647-Von Kohorn (continuous xanthation of crumb),3,385,845-Trieber (semi-continuous batch addition and passage throughdry xanthation stage followed by a final wet stage), 3,438,969-Meister(xanthation in a kneader-type reactor followed by a vertical column andsubsequent vertical solution-forming tower with agitator) and3,671,279-Yasui et al (continuous kneading and passage of crumb anddough-like material).

Of these, Robertson is most pertinent to the present invention in thatit involves a first dry xanthation stage and a second wet or immersionstage. In all of these "staged" processes, however, the CS₂ is fullyreacted and the xanthation reaction, with the available CS₂, is taken tocompletion in the first stage. This procedure neglects the sequentialpath of the xanthation reaction in which the secondary hydroxyls of thecellulose molecule (the conventionally designated C₂ and C₃ positions)are readily substituted in a matter of minutes while the transfer of thesubstituted CS₂ to the primary hydroxyl (at the conventionallydesignated C₆ position) takes much longer. The latter transfer, however,occurs much more readily in a slurry xanthate reaction.

Having in mind certain inherent disadvantages in the continuous orsemi-continuous xanthation processes and apparatus heretofore known, andin the processes and apparatus for the subsequent conversion of thexanthate product into a spinning solution, it is the general object ofthe present invention to provide an improved and more practical processand apparatus for continuously converting either shredded crumbs orcontinuous strips of alkali cellulose in sheet form into, ultimately, aviscose spinning solution.

A more specific objective of this invention is to provide process andapparatus facilitating in-line continuous xanthation and dissolution ofthe xanthated product in stages designed to effect maximum overallefficiency and minimum process problems associated with typical batchand semi-batch processes.

These objectives, and others which will be apparent in the course of thesubsequent discussion, are met, briefly, by a xanthation systemincluding a vessel containing a CS₂ -alkali hydroxide slurry in whichalkali cellulose feed, which has been first partially xanthated in a drystate for a short period of time, is immersed and held for a period oftime. Carbon disulfide is also introduced into the vessel at a rate toeffect at least partial xanthation of the incoming alkali cellulose feedat its pre-selected feed rate. Slurry is withdrawn from the bottom ofthe vessel at a rate to maintain a minimum residence time, on the orderof 5-10 minutes, of alkali cellulose in the vessel. Means are alsoincluded for introducing make-up alkali hydroxide solution to maintain aconstant level of liquid slurry in the vessel.

The process is generally carried out at atmospheric pressure with atemperature maintained in the range of 30°-45° C., preferably about 35°C. Slurry taken from the slurry holding vessel is transferred to a screwextruder mixer where further mixing and xanthation occur, with thefurther addition of carbon disulfide and alkali hydroxide solution asnecessary.

The alkali cellulose feed to the slurry vessel (where it is firstimmersed and reacted) is a shredded partially pre-xanthated alkalicellulose which is delivered through a closed passageway from a closedpre-xanthation chamber into which sufficient carbon disulfide isintroduced to effect the desired partial pre-xanthation. Means may alsobe included for introducing an indefinite length strip of alkalicellulose in sheet form in a continuous manner into such apre-xanthation chamber with sealing means to prevent escape of gasestherefrom at the alkali cellulose entry point. The alkali cellulose ispassed continuously through the chamber, where it is partiallypre-xanthated (and shredded, if in sheet form), and from there to theslurry xanthation vessel previously described.

The present invention also encompasses continuous mixing and spinningsolution-forming means particularly adapted to be used downstream of andin conjunction with the continuous xanthation process and apparatus ofthis invention. This continuous mixing means comprises generally anelongated passageway preferably including mass dividers or "attritors"near the upstream end thereof to break down the otherwise viscous stickymass of xanthated alkali cellulose passing therethrough. The elongatedpassageway also includes, at spaced intervals, means for introducingadditional alkali hydroxide solution followed by internal mixerscontinuously mixing the newly introduced alkali hydroxide solution withthe mass in the passageway. At least two such additional solutionintroducing means and internal mixing means should be spaced atintervals along the passageway. Downstream, a homogenous solution ofxanthated alkali cellulose and alkali hydroxide is delivered in formsuitable for aging, if required, filtration and spinning into fiberform.

This invention may be better understood by reference to the subsequentdetailed description thereof, taken in conjunction with the appendedclaims and the figures, wherein:

FIG. 1 is a diagrammatical illustration of the basic elements of the dryand slurry xanthation and viscose spinning solution forming apparatus ofthe present invention in its preferred embodiment; and

FIG. 2 is a diagrammatical illustration of one type of pre-xanthationand shredding stage preferably used in combining the xanthation processand apparatus of this invention with a continuous sheet steeping processfor producing alkali cellulose.

The xanthate reaction, that is the reaction between CS₂ and AKC, cantake place in the gas-solid phase or in the solution phase (that is, CS₂introduced into a slurry of AKC, NaOH and water). There are advantagesto be gained in both quality and speed of reaction by using bothreaction mechanisms. In the initial stages of the reaction it isadvantageous, both for final quality, and speed of complete xanthationand mixing, to utilize the gas-solid reaction, keeping the AKC "dry"i.e., undissolved. In less than five minutes, the CS₂ can be substitutedon the C₂ and C₃ hydroxyls (the secondary hydroxyls) in the drypre-xanthation stage. Subsequently the reaction of xanthation andsolution (including transfer of the CS₂ to the C₆ or primary hydroxylposition to complete the xanthation process) proceeds at a much higherrate if it is carried out in solution or slurry. In the process andapparatus of the present invention, maximum advantage is taken of bothof these reaction possibilities.

Turning more specifically to FIG. 1, there is shown a process andapparatus wherein shredded alkali cellulose or crumb 1 is fed into thetop of a cylindrical column 2. The AKC is advanced vertically down thecolumn by means of an advancing screw 3. Approximately 5 feet down thecolumn, CS₂ is introduced through several spray heads 4 placed aroundthe column. The height of AKC above the CS₂ spray heads 4 serves to sealoff CS₂ preventing it from escaping upward out of the apparatus as theAKC will absorb and react with all the CS₂ introduced. The xanthatereaction is thereby initiated in this region. The AKC-CS₂ mixturecontinues downward in the column for approximately five feet more whilethe xanthate reaction is progressing. Its total residence time in thispart of the reactor is on the order of 3-20 minutes and 5-35% of thetotal required CS₂ is introduced in this dry stage. The reacting mass isthen introduced into a larger diameter column called the primary reactor5. A second advancing screw 6 insures the uniform advancement of themass downward. However, this screw is designed to turn at a moderatespeed and to insure thorough blending and mixing throughout the reactor.At the head of this reactor additional CS₂ is added by injectors 7 anddirectly thereafter a water solution of alkali hydroxide, usually Na0H,is introduced through injectors 8. The AKC has by this time becomepartially xanthated and the solubility in weak alkali has increased tothe point that partial dissolution takes place. The dissolved andpartially reacted AKC forms a slurry in which the xanthate reactioncontinues and dissolution of the AKC-xanthate continues as it forms.

The CS₂ added in the crumb or shredded AKC column, and in the primaryreactor constitutes a large portion, if not all, of the total CS₂ chargerequired. In some cases a third portion of CS₂ may be added throughinjection to the secondary reactor, to be described later.

Near the top of the primary reactor there may be introduced arecirculated stream of nearly dissolved, and completely xanthated slurrythrough recycle line 10 from the end of the secondary reactor alsoincluding recycle slurry pump SP₁. The purpose of this recirculation isto take advantage of the fact that partially xanthated AKC dissolvesmore readily and uniformly in viscose solution than in a water solutionof Na0H or K0H.

From the primary reactor the slurry is continuously pumped, by slurrypump 11, into the secondary reactor. The rate of pumping is of courseequal to the rate necessary to remove cellulose from the AKC column 2 asit is introduced. The dwell time in the primary reactor 5 isapproximately 10 minutes. The secondary reactor 12 follows the samegeneral design as the primary reactor. However, the vessel is larger, inorder to achieve a dwell time of up to 60 minutes. NaOH and H₂ O areintroduced through inlet lines 13 at the top of the reactor 12, tofurther the progress of dissolution. A third portion of the total CS₂charge may in some cases also be added here through injectors 9 althoughin most instances the total CS₂ required will have been added in the AKCcolumn 2 and in the primary reactor 5. The advancing screw 14 insecondary reactor 12, as in the primary reactor, is designed to insureuniform advance and thorough blending of the chemical mass. From theoutlet of this reactor some viscose slurry may be recirculated to thebeginning of the primary reactor through recycle line 10 as discussedabove.

From the secondary reactor the slurry is pumped by SP₂ continuouslythrough a series of attrition mixing chambers 15-16. The viscose slurryis passed alternately through attrition mills 16 and then mixed by highspeed blending propellers 17. Fractions of the total caustic solutioncharge are added through inlet lines 18 in various cells 15.

It is well known that the most efficacious way to bring viscose intofinal solution is to do most of the mixing and kneading while the slurryis thick and pasty. After a homogenous, essentially dissolved,concentrated solution is achieved the final addition of water andcaustic are added to bring the solution to the desired finalcomposition. In this way maximum homogeneity is achieved with a minimumof undissolved gels, in minimum solution time.

Heretofore, it has not been practicable to take advantage of thisprinciple as batch xanthate and mixing processes were not well adaptedto use the techniques required.

In this process, xanthation and mixing is achieved in about 90 minutesor less by a continuous process requiring little direct labor.Furthermore, the viscose is expected to be of better quality andspinability than any heretofore achieved commercially. This process hasbeen shown in its preferred form with vertical columns and advancingscrews. However, horizontal vessels with equivalent advancing mechanismsmay also be used.

Turning now to FIG. 2, there is shown a prexanthation stage in which acontinuous sheet of AKC is partially xanthated and shredded prior topassage to the xanthation process and apparatus shown in FIG. 1. Morespecifically, a continuous sheet of mercerized alkali cellulose 21 isfed continuously by means of feed rolls 22 into an enclosed chamber 23.Chamber 23 may be connected to a carbon absorption-type CS₂ recoverysystem conventionally used in the viscose process for recovering dilutemixtures of CS₂ gas from air. If desired this chamber may contain anitrogen atmosphere.

The advancing sheet of AKC is fed over "kiss" rolls 24 whereby CS₂ ispicked up from a feeding system 25 and imparted to the AKC sheet. Thesheet is then passed through disintegrator 26 and the shredded crumbs 27are fed into the AKC column 2 of FIG. 1. Alternatively, instead of using"kiss" rolls, a spray of CS₂ may be injected onto the sheet from eachside. CS₂ injectors 4 (of FIG. 1) may be omitted if the drypre-xanthation is accomplished in a chamber such as that shown in FIG.2.

The following examples typify proposed conditions and expected resultsin the possible practice of the present invention.

EXAMPLE 1

The feed to the continuous xanthator is an alkali cellulose, by weight:

33% cellulose,

15% Na0H,

52% h₂ o.

this AKC is pre-shredded, and is fed to initial xanthate and seal column2 at a rate of 150 pounds per minute. The following total amounts ofmaterials are added throughout the reaction:

14 pounds CS₂ per minute

380 pounds H₂ O per minute

11 pounds Na0H per minute

to produce a final desired viscose which has the composition:

28.0% CS₂, based on dry cellulose weight,

9.0% cellulose, based on viscose weight,

6.0% NaOH, based on viscose weight,

82.5% H₂ O, based on viscose weight.

Portions of CS₂ are added at the points indicated as follows:

    ______________________________________                                        Point          CS.sub.2 (pounds added per min)                                ______________________________________                                        4 FIG. 1       4.7                                                            7 FIG. 1       4.7                                                            9 FIG. 1       4.6                                                            ______________________________________                                    

Portions of H₂ O and NaOH are also added at the places and in amounts asfollows:

    ______________________________________                                                         (pounds added)    (pounds added)                             Point     NaOH   per minute   H.sub.2 O                                                                          per minute                                 ______________________________________                                        8 FIG. 1  5.5                 48.4                                            13 FIG. 1 5.5                 48.4                                            18a FIG. 1                                                                              0.0                 94.4                                            18b FIG. 1                                                                              0.0                 94.4                                            18c FIG. 1                                                                              0.0                 94.4                                            ______________________________________                                    

The dwell times, and operating temperatures for the various stages ofthe process, as shown in FIG. 1, are given in the table below:

    ______________________________________                                        Reaction Stage                                                                             Dwell Time                                                       or Position  (minutes)    Temperature(° C.)                            ______________________________________                                        Incoming AKC FIG. 1       40                                                  Reactor 2 FIG. 1                                                                           15           40 - 35                                             Reactor 5 FIG. 1                                                                           10           35                                                  Reactor 12 FIG. 1                                                                          35           35                                                  Attritor-Mixer                                                                             30           30                                                  16a, 16b, 16c,                                                                18a, 18b, 18c                                                                 ______________________________________                                    

EXAMPLE 2

The alkali cellulose feed in this example comes from a continuous sheetdouble-steep-mercerizing system.

AKC feed to xanthator - mixing process (FIGS. 1-2)

10% naOH,

33% cellulose,

57% H₂ O.

The final desired viscose is, as in Example 1:

28% CS₂, based on dry cellulose weight,

9% cellulose, based on viscose weight,

6% NaOH, based on viscose weight,

82.5% H₂ O, based on viscose weight.

Total chemicals added through the xanthate-mixing process:

14.0 pounds CS₂, per minute

18.3 pounds NaOH, per minute

373.0 pounds H₂ O, per minute

Portions of CS₂ are introduced at the placed indicated as follows:

    ______________________________________                                        Point         CS.sub.2 (pounds added per minute)                              ______________________________________                                        24 FIG. 2     4                                                               7 FIG. 1      5                                                               9 FIG. 1      5                                                               ______________________________________                                    

Portions of NaOH and H₂ O are added at the places and in the amountsindicated, as follows:

    ______________________________________                                                         (pounds added)    (pounds added)                             Point     NaOH   per minute   H.sub.2 O                                                                          per minute                                 ______________________________________                                        8 FIG. 1  9.2                 82.5                                            13 FIG. 1 9.1                 82.5                                            18a FIG. 1                                                                              0.0                 69.0                                            18b FIG. 1                                                                              0.0                 69.0                                            18c FIG. 1                                                                              0.0                 70.0                                            ______________________________________                                    

The dwell times and operating temperatures for this example are asfollows:

    ______________________________________                                        Reaction Stage                                                                             Dwell Time                                                       or Position  (minutes)    Temperature(° C.)                            ______________________________________                                        Chamber 23 FIG. 2                                                                          1            35                                                  Reactor  2 FIG. 1                                                                          15           35                                                  Reactor  5 FIG. 1                                                                          10           35                                                  Reactor 12 FIG. 1                                                                          35           35                                                  Attritor-Mixter                                                                            30           35                                                  16, 18 FIG. 1                                                                 ______________________________________                                    

In each example, the temperature of the entering AKC may be varied from30° to 60° C., depending upon method of mercerization. The preferredtemperature in the primary and secondary reactors is 35° C., but mayvary between 30°-45° C.

While this invention has been described with reference to xanthation inthe presence of an alkali hydroxide solution generally, normally thissolution is sodium hydroxide. Potassium hydroxide may also be used.

Various other such minor modifications and variations of the presentinvention will be apparent to those skilled in the art. While thisinvention has been described, for purposes of definiteness andspecificity as required by the statute, with respect to specificembodiments and examples, it should be understood that it is not limitedthereto and that the appended claims are therefore intended to beconstrued to encompass those various modifications and variations whichmay be made in this invention without departing from the true spirit andscope thereof.

We claim:
 1. In a process for xanthating alkali cellulose by contactingsaid alkali cellulose with carbon disulfide, the improvement consistingof continuously feeding alkali cellulose at a predetermined rate firstin dry form to a vessel where it is brought into contact for 3-20minutes with 5-35% of the CS₂ necessary to provide partial xanthation ofthe cellulose and then, prior to completion of reaction between said CS₂and said dry cellulose, passing said cellulose on to a vessel containingan alkali hydroxide slurry, immersing said cellulose therein whilecontinuously introducing carbon disulfide into said vessel at acontrolled rate, continuously feeding alkali hydroxide solution intosaid vessel at a controlled rate, and continuously withdrawing a portionof said slurry from the bottom of said vessel at a rate, selected tomaintain a preselected minimum residence time of alkali cellulose insaid vessel, then passing said withdrawn slurry through a screw extrudermixer, introducing further carbon disulfide and alkali hydroxidesolution thereto, as required, to effect further xanthation therein. 2.An improved process, as recited in claim 1, wherein said minimumresidence time in said slurry vessel is 5-10 minutes, the temperature ofsaid slurry is about 35° C. and the pressure in said vessel is about 1.0atmospheres.
 3. In improved process, as recited in claim 1, wherein aportion of the xanthated alkali cellulose product from said screwextruder mixer is recirculated to said slurry-containing vessel.
 4. Animproved process, as recited in claim 1, wherein said alkali hydroxideis sodium hydroxide and said alkali cellulose is sodium cellulose.
 5. Animproved process, as recited in claim 1, wherein said dry partiallyxanthated alkali cellulose is in strip form and passes through a closedpassageway from a sealed chamber to said slurry-containing vessel.
 6. Animproved process, as recited in claim 5, wherein said alkali cellulosestrip is shredded after it passes through said sealed chamber and beforeit enters said slurry-containing vessel.
 7. An improved process, asrecited in claim 1, wherein said alkali cellulose is shredded and passesdownwardly into a vessel containing gaseous carbon disulfide, saidincoming alkali cellulose capturing and preventing said carbon disulfidefrom escaping from the top of said vessel.